1. Field of the Invention
The present invention relates to a surface light source device that converts linear emission light, which is emitted from a light source, to planar emission light.
2. Description of the Related Art
A prior-art surface light source device includes a cold-cathode fluorescent lamp that is a discharge lamp serving as an elongated cylindrical linear light source that emits linear light. The cold-cathode fluorescent lamp is surrounded by a sheet-like lamp reflector that serves as a reflecting member. The lamp reflector has an opening portion, through which emission light from the cold-cathode fluorescent lamp is taken out. The opening portion of the lamp reflector is fitted on the side surface of a light guide that converts linear light from the cold-cathode fluorescent lamp to planar light and emits the planar light from the surface thereof. Prisms are formed on the back surface of the light guide.
In this surface light source device, emission light from the cold-cathode fluorescent lamp is reflected by the lamp reflector and is guided to the side surface of the light guide through the opening portion. The light that is guided to the light guide is emitted from the surface of the light guide. The emission light from the surface of the light guide is collected or diffused by an optical sheet that is disposed on the front surface of the light guide. Part of the light that is guided to the light guide emerges from the back surface of the light guide. The emission light from the back surface of the light guide is reflected by an optical sheet, which is disposed on back surface of the light guide, toward the light guide side (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 2002-270024).
In recent years, there has been a demand for still higher luminance, a further reduction in thickness and weight and a further reduction in cost in the liquid crystal display that includes this type of surface light source device. Hence, still higher luminance, a further reduction in thickness and weight and a further reduction in cost are required in the surface light source device. The provision of the prisms on the back surface of the light guide is an effective measure for meeting the above demand, since the light collecting function is improved and the number of optical sheets can be reduced.
However, in the prior-art surface light source device, a slight gap may possibly occur between the lamp reflector and the light guide due to non-uniformity in shape of the lamp reflector or non-uniformity in assembly of the lamp reflector and light guide. As a result, emission light from the cold-cathode fluorescent lamp may enter the light guide from a surface thereof, which is other than the side surface thereof. To be more specific, emission light from the cold-cathode fluorescent lamp may leak from the gap between the lamp reflector and light guide and may enter the light guide from the front surface thereof. The light, which is incident on the light guide from the surface, other than the side surface, of the light guide, may cause dark-luminance lines or luminance non-uniformity on the front surface of the light guide. In addition, when the light guide that is provided with the prisms on the back surface thereof is employed, it is more likely that, by the light collecting function of the prisms, luminance non-uniformity may occur due to the light that enters the light guide from the surface, other than the side surface, of the light guide.
In short, in the prior-art surface light source device, there is the problem that an improvement in quality of in-plane luminance is not easy.